Electro-luminescent display devices



g- 2, 1960 G. R. HOFFMAN ET AL 2,947,912

ELECTRO-LUMINESCENT DISPLAY DEVICES 2 Sheets-Sheet 1 Filed April '7, 1958 '5 I 0 I! I I IW/ P ZQ: I H0 5- f W a I5 I 0 V I0 i 0 V fi I IZ IIZ Il Il IIO IIV I mI +V 1 1612* f j Aug. 1960 G. R. HOFFMAN ETAL 2,947,912

ELECTRO-LUMINESCENT DISPLAY DEVICES Filed April 7, 1958 2 Sheets-Sheet 2 34 2V Z9, S/GNAL- I T CONTROLLED SWITCH A) J] ,5? k OPERATING MEANS 3M 2 I be. Rwk d- "Hr h m bmwl km,

S W/ TCH OPERA T/NG ME A NS 2,941,912 7 ELECTRO-LUMINESQCENT DISPLAY DEVICES 1 George Richard Hoffman, 'Sale, and Roy Ernest Hayes, Cheltenham, England, assignors to National Research Development Corporation, London, England, 'a British corporation Filed'Apr. '1, 1958, .SenNo. 125,832 Claims priority, application Great Britain Apr. 9, .1957

' 11 Claims. Cl. 315-469); V

This invention "relates to a display device usable instead of a cathode ray tube and employing the phenomena of electro-luminescence under the influence of a changing electric field. L i f Normal cathode ray tube devices have limited accuracy in the positioning of the focused light 'spot formed on the fluorescent screen of the'tube'by the impact therewith of the electron beam. Moreover, the deflection sensi- Patented Aug. 2, 1960 of the first conductors 11 and a similar individual connecting lead is provided for each of the second conductors 13; These leads may be joined to the conductors themselves by use of any suitable one of the already well known methods.

If any selected conductor of the first set 11 is now energised with a' voltage V of one polarity and any selected conductor of the second set 13 is similarly energised with a voltage V of oppositepolarity, then at theintersection point of the selected conductors electrotivity of the produced light's pot varies over the surface area of the screen with resultant barrel or pin-cushion distortion and precise repetition of any particular spot position is difliculfunless' expensive drift-free stabilisation techniques are employed in the associated apparatus.

The object of the present inventionjis to provide a dis: play device capable of use as an alternativeto a cathode ray tube and in which the above limitations of of cathode ray'tube-devices are avoided, v v

In the device according to thebroadest aspect of' the invention a'layer of suitable phosphor is caused to fluoresce by the application thereto of a'cha'nging electric field produced between a selected one of a first set of electric conductors disposed on one side of the phosphor layer and a selected one of a second set of. electric con,- ductors which are disposed on the opposite side of such phosphor layer. Preferably the respective conductorsjof the two sets heat right angles toi 'each other to form 'aj matrix. 7 p

In order that the nature of the inventionmaybe more readily understood several embodiments thereof will now be described with reference to the accompanying"drawin'gs,inwhich:' Y

Fig. 1 is a fragmentary perspectiveview of one formj of electro -luminescent device according to the. invention: Fig. 2 consists of diagrams (a) and (b) illustrating* respectively a suitable energising waveform'and the resultantlight output produced thereby. 1 J

"Figs. 3 and 4 are diagrams of small; parts of the conductor matrix shown to a much enlarged scale, Fig. 5 is an exploded perspectivefview illustrating the manner of manufacture of an alternative embodiment of the invention. I

Fig. 6 is a schematic diagram showing one display device arrangement according to the invention."

-In the constructional form of display deviceifo'rj use, in-the invention illustrated in-Fig. 1 a glass plate10f first has deposited thereon a' suitable number, for instance 1,000, of parallel and, preferably, I transparent"first .electric conductors 11. These conductors are conveniently operation a is a constant,

luminescence of the phosphor layer 12 is produced. This can be viewed through the glass plate 10. By arranging for the sequential selective energisation of the different conductors of the two sets so a graphical or other'figure can be displayed.

As electro-lurninescence is a function of a changing electric field it' is normally necessary to employ a fluctuating energising potential on each conductor and this is conveniently in the form of a voltage square wave. The

resultant light 'outpu't for a square Wave energising input as shown in Fig. 2(a) is indicated approximately in Fig.

2(b) and with many useful phosphors the time constant of the light output decay is of the order of 10 microseconds' In consequence of this, the integrated light output increases substantially linearly with increase of frequency of the appliedvoltage square Wave until the period time of such wave is of the same order as the decay time. A kc./s. square wave has been found to be a reasonable frequency for the applied energising square wave voltage. i

The integrated light output from such a device varies -with the voltage applied and over a limited rangef can be expressed by the simple law L =aV where p L is the integrated light output Y the voltage applied and i i V n is' a factor of the order of 8. V f

In practice, with an example constructed as described above, a change of applied voltage between intersecting first and second conductors 11 and 13 from a first value of 150 v. to a second .value of .300 v. caused an integrated light output change of the ratio of 1:300.

A device of the .char'acter' described may readily be adapted to provide a graphical display, such as that of a curve or complex line figure, by suitably energising each in the form of very thin gold-film'bars' 0.00l-inchwide and. are equally spaced =50 to'the inch" The deposition of these gold film conductors can be by' any"alreadywell of one 'set of conductors, for instance, the set of condoctors 11 in a regular cyclic manner whilst controlling the selective connection of the conductors of the other set 13 to the energising voltage source bymeans respon sive to the variable factorwhich is to be displayed. Thus, one set of conductors such as those which are disposed to lie vertically on a display panel may be energised in turn through asuitable time-controlled selector switch while the o'pposite set of horizontal conductors. may be connected to their energising sourcev by way -of a relay tree, the form of operation of which is continuously controlled by binary input signals representing the variable value which is to be'disp-layed to the time scale set by the aforesaid selector switch.

a thin uniform layer 12 of electro-lurninesc'ent phosphor-I V The phosphor material employed may be any 'of those already well known in connec tion elecrtro-fluores- Such anarrangement is particularlysuitable for ob tainingphotogi'aphic records-byrneans of a suitable light cence phenomena.-' Thereaftenla number offurther or second conductors 1 3, for inst-anee'LOpQ-arethen i t sensitive plate disposed adjacent the display element or subjectedto the light therefromthrough a suitable lens' system.

One difliculty which arises, however, with any arrangement, such as photographic recording, where integration of the total light output from any one electro-luminescent point is efiected, is demonstrated as follows. If, as would usually be the case, all of the conductorswhich are unused at any given instant are earthed, then along each of the selected conductors there is a voltage difference. V at each of the intersections of that conductor with the unused and earthed opposing conductors as well as the. desired voltage difierence 2V at the intersection of the two 9 selected 7 conductors where electro-luminescence is desired. This is illustrated in Fig. 3 where the various intersection points abetwcen the selected conductor 13 of" the set of conductors 13 and the unused and earthed conductors of the set of conductors 11 provide a voltage difierence V and the similar intersection points b between the selected conductor 11? and the unused and earthed conductors leans provide a voltage difference V Point at the intersection of conductors 11 and 13* provides the required operative voltage diflierence 2V while at the remaining intersection points d of the earthed conductors of each set there is, of course, zero voltage difference.

Consider. now an extreme case where each of 1,000 conductors 11 are energised in turn to provide, for example, a horizontal time scale, and where the variable quantity to be displayed is constant at the level of one conductor 13 forv the whole time period of the horizontal scale except for one instant when, say,at the time instant corresponding to energisation of conductor 11*, the quantity value alters to require energisation of'conductor 13 This should produce a record having no luminescent image at point c and an image at point c Owing, however, to the otherwise continuous energisation of conductor 13 the phosphor at intersection point c will have been subjected to the voltage dilference V (i.e. that with an earthed unused conductor) 999- times whereas the wanted display point e will have been subjected to the voltage difference 2V only once. Under the above noted law governing integrated light output, the integrated light output from point c will be 300 units whereas. that from point c will be 999 units. In consequence the required; display point 0 will be obscured by the unwanted display pointc.

In order to overcome this difiiculty and in accordance with another feature of the invention, arrangements are provided for supplying to each" ofthe conductors of each set which are unused at any time a bias voltage and having a polarity opposite to that with which the same set of conductors is energised to produce display.

Thus, and as shown in Fig. 4, taking the same conditions.

Applying the above quoted law, the difierence in integrated light output between a point suchas c and points such as a, ord ncw becomes proportional to In consequence, in the extreme case previously discussed.

the ratio of the respective integrated light outputs, ofi points c and c is of the order of 6,500 to 999 or,greater] thanl6z1. This is amply s'uflici ent to provide adequate discrimination.

wens- It w ll 2 ea erness ha the atq ai bias voltages are each one-third amplitude versions of the fluctuating, e.g. square wave, voltages used on the selected conductors. Although a square wave voltage is preferable, a sine waveform may be used and is more. conveniently obtainable.

By suitable selection of exposure and development techniques in photographic recording, it is possible to increase the contrast of the wanted to unwanted spot positions and the exposure threshold of the photographic material employed 'may readily be exploited.

Fig. 6 illustrates, in schematic diagram form, one arrangement for operating with the above described biased system of the unused conductors. For simplicity the matrix is shown with only five first conductors L1 and four second conductors 13 but it will be understood that many more conductors may be used in each set with appropriate modification of the associated switching means.

Each of the first conductors 11 is connected by way of leadslq to. individual contacts of two multi-point selector switches 20, 21. The rotors 22, 23 of these switches are ganged together for operation in unison by means 24 which may be atime-controlled or synchronous motor. The rotor 23' of switch 21 is arranged to make connection with only one of the switch contacts at any one time whereas the rotor 22 of'switch 20 is arranged to bridge all of the switch contacts except that which is joined in parallel with the particular contact of switch 21 engaged by rotor 23 at the particular switch position in use.

Each of the second conductorsv 13 is likewise connected by way of leads 1 5, to, individual contacts oftwo further multi-point switches 25, 26. The rotors 27, 28 of these. switches. are also ganged for operation in unison by signal controlled means. 29 whereby the switches 25, 26. may be set in any one. of the, various alternative positions under the control of an. input. signal on lead 30. Therotor. 2 8. ot switch 26. is. arranged to make connection with only one, ofthefswitch. contacts atany. one time whereas the rotor 27 of switch 26 is arranged to bridge. all of the. SWltChzCOHtB-C'ES. except that which is joined in parallel with theparticular. contact of Switch 2 6 engaged by. rotor 27. at theparticular. switchposition in use.

A source of alternating potential 30, e.g. a square orsine. wave,generatoroperating ata frequency of'the order or 50. kc./s.' and providing an alternating potential out-.

put. whichis balanced with respect'to earth, has its respect i ve output leads. 31, 32 connected -to opposite ends of a 31 and 32. Lead 37 is connected to rotor 27 of switch- 25.. whereas lead 38 is connected to. rotor 22 of switch 20.

By appropriate operation ofthe selector switch means constituted by the switchpairsjt), 21 and 25, 26 any desiredone of-the firstconductorsltand of. the second conductors. 13 may be. connected respectively across the. full.

potential 2V ofthe sourcefilkwhile each ofthe remaining. unselected; conductors 11 and 13 are connected.- re spectively to.. the one-thirdvaluepotential point of polarits? opposite thatwithwh qhthe related-selected conductor:

1 1', 'lig ssu pli a Whileth'e selector switch meansghave. beenshown as.

oi siiriple-mechanical switch form, it will be understood that. any suitable electronic .or other high-speedswitching means maybe employed in lieu.

In the construction of devices described above certain problems arise andthe first of these is the making of the individual connections 14 and 15 to .each of the;

or dragging a casting knife across the surface have so far been found to provide the best solution.

In an alternative construction of display device according to the invention each of the two sets of conductors are; opaque and only the light available around the edges of one of the sets of conductors is available for display purposes. In spite ofthis disadvantage, the increased facility for construction is in many circumstances more advantageous. One construction ofthis. nature is illustrated in Fig. 5 and comprises a pair of rectangular frames 20, 21, e.g. of metal. One surface 22 of each frame is ground flat. Each frame is then provided with a series of accurately spaced wires 23, 24 conveniently located and held in a series'of notches along edges of the frames. The two frames with their attached wires are then brought together with their ground surfaces 22 facing each other and spaced apart at an accurately determined distance by means of inserted metal, shim plates 25. A glass plate 26 to support the finished structure is then placed adjacent to the underside of the bottom set of wires 23 and phosphor material (not shown). is persuaded to encompass the matrix of parallel wires by spraying, dipping or the like.- With this method the conductors are accurately spaced and the difliculty of providing connecting wires to the various conductors of the matrix is avoided. At least the upper or outer set of wires are preferably very thin, conveniently of the order of 0.003 inch diameter which has been found sufficient to produce reasonable fluorescence at the wire edges and still allow visibility of the electro-luminescence produced. With such a device the photographic recording material may be laid directly adjacent the surface of the phosphor with consequent decrease of the exposure time necessary.

We claim:

1. An electro-luminescent display device comprising a phosphor layer, a first set of electric conductors disposed on one side of said layer, a second set of electric conductors disposed on the opposite side of said layer in non-parallel relationship to said first set conductors whereby each second set conductor crosses a plurality.

of said first set conductors, a source of varying electric operating potential, a source of varying electric biasing potential having at all times a predetermined and constant fractional value which is between one half and one quarter of that of said source of operating potential, said sources of potential each having first and second terminals, said first terminals being of like polarity at any instant and said second terminals being also of like polarity at any instant but of opposite polarity to said first terminals and selector switching means including equally conductive bi-directional connections for connecting the first terminal of said operating potential source to one of said first set conductors and the second terminal of said operating potential source to one of said second set conductors and simultaneously connecting the second terminal of said biasing potential source to each of said first set conductors which is not connected to said operating potential source and for connecting said first terminal of said biasing potential source to each of said second set conductors which is not connected to said operating potential source.

2. An electro-luminescent display device comprising a phosphor layer, a first set of parallel linear electric conductors disposed on one side of said layer, a second set of parallel linear electric conductors disposed on the opposite side of said layer at right angles to said first set conductors whereby each second set conductor crosses each of said first set conductors, a source of alternating,

electric operating potential, a source ,of alternating electric biasing potential having at all times'avalue which is one third that of said sourceof operating potential, said sources of potential each having first terminals which'sare. of like polarity at any instant and second terminals. which are ,of like polarity at any instant and selector switching means including equally conductive, bi-directional connections for connecting the first terminalv of said operating potential source to any chosen oneof said first setconductors and the'second terminal of said operating'potential source to any chosen one of said second set conductors and simultaneously connecting the second terminal of said biasing potential source to each of said first set conductors which is'not connected, to

said operating potential source and for connecting said first terminal of-said biasing potential source to each of,

saidQsecond set conductors which is not connected to said operating potential source.

3; An electro-lurninescent display device comprising a planar layer of electro-luminescent phosphor material, a plurality of parallel first electric conductors disposed on and in contact with one side of said phosphor layer, a plurality of parallel second electric conductors disposed on the'opposite side of and in contact with said phosphor 1ayer, said second conductors being disposed at right angles to said first conductors, a source of varyingvelec-j tric operating potential having first and second terminals of respective polarities which are opposite to each other at any given instant, asource of lvarying'electric biasing potential haying first and; second terminals, which are respectively of opposite polarity to each other at any instant and of the same polarity as said first and second terminals of said operating potential source at any instant, said source of biasing potential being at all times one third the value of said operating potential and selector switch means including equally conductive bi-directional connections for connecting said first and second terminals respectively of said operating potential source to a selected one of said first conductors and to a selected one of said second conductors and for simultaneously connecting all of the remaining first conductors to said second terminal of said biasing potential source and for connecting all of the remaining second conductors to said first terminal of said biasing potential source.

4. An electro-luminescent display device according to claim 3 in which the varying potential applied between said first and second conductors is of alternating character with an alternation frequency of the order of 50 kc./s.

5. An electro-luminescent display device according to claim 4 in which the varying potential applied between said first and second conductors has a square waveform.

6. An electro-luminescent display device comprising a planar layer of electro-luminescent phosphor material, a plurality of linear and parallel first electric conductors disposed on and in contact with one side of said phosphor layer, ,a plurality of linear and parallel second electric conductors disposed on the opposite side of and in contact with said phosphor layer, said second conductors being disposed at right angles to said first conductors, a source of alternating electric operating potential having first and second terminals of respective polarities which are opposite to each other at any given instant, a source of alternating electric biasing potential having first and second terminals which are respectively of opposite polarity to each otherat any instant and of the same polarity as said first and second terminals of said operating potential source at any instant, the voltage of said biasing potential being at all times one third the voltage of said operating potential and selector switch means including equally conductive-bidirectional connections for connecting said first and second terminals respectively of said operating potential source to a selected one of said first conductor's and to a selected one of said secondeach in the form of thin metal wires embedded within a body of phosphor material forming said phosphor 1ayer.

I 8. An electro-luminescent display device according to claim 6 in which saidselector switch means include a first cyclically operable switch for connecting each of said first conductors in turn to said operating potential source and a second signal-controlled switch for connecting any chosen one of said second conductors to said operating potential source in dependence upon the nature of a control signal applied to said second switch.

9. An electro-luminescent display device according to claim 8 in which said first cyclically operable switch comprises time-controlled switch operating means.

10. An electro-luminescent display device according to claim 8 in which said second signal-controlled switch comprises a relay tree. a

11. An electro-luminescent display device comprising a planar layer of. electro-luminescent phosphor material, a plurality oflinear and parallel first electric conductors disposed-on and in contact with one side of said phosphor layer, a plurality of parallel second electric conductors disposed on the opposite side of and in contact with said phosphor layer, said second conductors being transparent and disposed at right angles tosaid first conductors, a source of alternating electric operatingpotential: having first and second terminals of respective polarities which are opposite to each other at any given" instant, a source ofalternating electric biasing poten tial'having first and secondterminals which are respectively of opposite'polarity to each other at any instant and of the same polarity as said first and second terminals of said operating potential source at any instant, the voltage of said biasing potential being at all times one third the voltage of said operating potential and selector switch means including equally conductive bi-directional connections for connecting said first and secondterminalsi respectively of said operating potentialsource to a selected one of said first conductors and a selected one of said second conductors and for simultaneously connecting all of'the remaining first conductors to said second terminal of said biasing potential source and for connecting all of the remaining second conductors tosaid first terminal of" said biasing potential source.

References-Cited in the file of this. patent UNITED STATES PATENTS 2,774,813 Livingston Dec. 18, 1956 2,796,584 Hurvitz June 18, 1957 2,818,531 Peek Dec-31, 1957 2,877,371 Orthuber et al; Mar. 10, 19 59 2,881,360 Livingston Apr. 7, 1959 2,883,582 Hamlet Apr. 21, 1959 2,892,968 Kallman'et a1 June 30, 1959 

